1. Field of Invention
The present invention relates generally to networks. More particularly, the present invention relates to a multiformat transmitter that may be used in a network to select a transmission format such as either a non-return to zero transmission format or an optical duobinary transmission format as appropriate given performance requirements within the network.
2. Description of the Related Art
A non-return to zero (NRZ) modulation format is a typical modulation format that is used in networks such as deployed networks. A NRZ modulation format is a format in which digital data transmission uses binary low and high states that could be transmitted through electrical or optical media. Optical NRZ transmissions are often characterized by poor dispersion robustness, thus requiring dispersion compensation techniques to improve NRZ modulation format robustness to the total amount of chromatic dispersion residual associated with the deployed networks.
In lieu of implementing a dispersion compensation technique to overcome the relatively poor dispersion robustness associated with an NRZ modulation format, an optical duobinary (ODB) modulation format may be used in a network. The use of an ODB modulation format may improve the application range of an uncompensated network. An ODB modulation format is generally cost effective, relatively easy to implement, and achieves large group velocity dispersion. However, when low chromatic dispersion residual values are present, an ODB modulation format is characterized by a poorer performance than a NRZ modulation format. Hence, the use of an ODB modulation format does not always result in a better performance than the use of a NRZ modulation format.
In order to improve the performance associated with deployed networks, electronic dispersion compensation (EDC) may be applied both to an ODB modulation format, in presence of low chromatic dispersion residual values, and to a NRZ modulation format, when dealing with high dispersion residuals. The use of EDC may be effective, but EDC is not always reliable. Further, compensators that implement EDC consume a relatively high amount of power, and incorporating EDC with a forward error correction (FEC) algorithm is often complicated.
Therefore, what is needed is a readily implemented, relatively inexpensive system which allows for a good performance for a given modulation format over a range of chromatic dispersion residual values. That is, what is desired is a method and apparatus which allows the benefits associated with both an ODB modulation format and a NRZ modulation format to be realized in a single transmission system.